SUMMARY A theoretical analysis of the mechanical stability of a twobladed cantilever helicopter rotor, whose hub is mounted upon isotropic elastic supports in the horizontal plane and whose elastic blades employ fixed root attachments, has been made and is reported here. This case is a generalization of a problem treated by Feingold in reference 1 where the blades are assumed inelastic and attached to the hub by means of vertical hinges. It is found that Feingold's solution remains applicable also to the general case if the Coleman parameters Ai, A2, and A3 are appro­ priately defined by using the natural frequencies and modal shapes of the free uncoupled lag bending vibrations of the blades. The numerical determination of Coleman's parameters for a typical nonuniform two-bladed cantilever rotor are given in Appendix A. The effects of root tension straps are treated in Appendix B.